test(playground): smoke-test the playground's own server plugins

[IamGalymzhan]

Stack

• #530 — the testing kit (merged)
  • #540 — createTestApp and the mock client
    • #555 — withEnv, context options, and createApiError
      • #542 — the playground smoke tests ← you are here

Each PR targets the one above it, so the diff shown here is only the delta on top of #555. Review in order.

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Summary

GitHub shows only this PR's 2 files here, and retargets automatically as the parents merge.

The playground's server plugins had no tests at all. apps/dev-playground/tests/ holds Playwright specs that intercept /api at the browser boundary (page.route + route.fulfill), so the Express server never runs in them — route handlers, execute() interceptors, and executeStream are entirely unexercised. These five tests cover that side with the harness from #540: real HTTP, faked data plane, no workspace, no credentials, no network.

It also serves as dogfooding that a customer would recognise — the reference app testing its own plugins the way the docs tell them to test theirs.

What's covered

Test	Proves
all three boot together	the plugin set composes and registers under its manifest names
GET /api/reconnect	the plain JSON route answers
reconnect stream	opens as text/event-stream and emits its first message
POST /api/telemetry-examples/combined	the whole nested-span body runs against the real TelemetryProvider and threads userId through
lakebase-examples unconfigured	degrades to no routes without PGHOST/LAKEBASE_ENDPOINT, and the app still boots

Adds a dev-playground vitest project so pnpm test and CI's Unit Tests job pick them up.